This invention relates to a tube sealing device with a cylindrical jacket which can be pressed areally against the inner wall of the tube. The jacket is made of a flexible material impermeable to liquids and gasses, and it has an end partition facing the pressure source and an end partition facing away from the pressure source. The tube is provided with a circular partition made of material impermeable to liquids and gasses which seals the jacket tightly at the end away from the pressure source.
A sealing device as described in U.S. Pat. No. 3,459,230 for a tube of the type mentioned is constructed as a sealing pad which, in addition to the named end partition, has an additional end partition which together provide a cylindrical inner space. If the inner space is filled with a gas or a liquid under pressure, the cylindrical jacket, especially in its middle area, bulges out radially and the jacket takes on a somewhat barrel shape. The diameter of the frontal end partitions remains constant during the filling. Near the two frontal end partitions, the outer diameter must be a little smaller than the inner diameter of the tube so that the device, which in its unpressurized state is essentially cylindrical, can be inserted into the tube.
This of necessity has the result that the sealing pad lies against only a part of the total length of the jacket along the inner wall of the tube. Near the end partitions there always remains a gap between the jacket and the inner wall of the tube. This gap tapers, starting at the end partitions, like a wedge towards the other end partition. The effective length of the gap, where the sealing pad actually touches with its jacket on the inner wall of the tube is, therefore, given by the geometric axial length of the jacket less the length of the two wedge-shaped gaps near the end partitions.
A sealing pad filled, for example, with air at 1.5 bar does not abut between these two wedge-shaped gaps with its full pressure against all places along the inner wall of the tube. At one place, where the gap decreases to nothing and the jacket touches the inner wall of the tube, the pressure is practically zero. The pressure increases from this point continuously up to the middle of the pad, and has, at least in the middle of the pad, but in general over a certain part of the axial length of the pad, the value of the inflation pressure. The pad in the example, inflated at 1.5 bar, thus abuts along only a part of its effective length, which is also shorter than its geometric length, with a pressure of a 1.5 bar against the inner wall of the tube.
The normal force with which a filled sealing pad abuts against the inner wall of a tube is determined essentially by the product of its inflation pressure and a fraction of the surface of its outer jacket. The disadvantage thereby is that the total surface of the outer jacket is not effective, and also that the normal force is not dependent on the acting counter force, that is, the pressure of a fluid or gas to be sealed out. In practical use, on the one side of the sealing pad there is found fluid under pressure, or gas under pressure. If the pressure of the medium to be sealed out is greater than the atmospheric pressure, then on the load side of the pad the gap next to the outer wall becomes longer. Expressed in a different way, the place where the outer jacket abuts with no pressure against the inner wall of the tube moves towards the center of the pad. This lengthening of the gap along the sides and the splitting effect impair the function of the sealing pad in a disasterous way: The higher the counter pressure to be sealed out, the longer the gap along the sides, and therefore, the shorter becomes the actual effective length of the device. In this way, with increasing counter pressure the normal force with which the sealing pad presses against the inner wall of the tube is decreased.
It would be desirable if the opposite were true, that is, that the normal force would increase with increased counter pressure. This is, however, not the case with the known sealing pads. As stated above, the inner pressure determining the normal force in the sealing pad is a value assumed to be existing, and is independent of the counter pressure and that part of the outer jacket which abuts against the inner wall of the tube. Thus the abuting force becomes smaller as the counter pressure increases.
The result is that a pad of the construction described above cannot withstand counter pressures which are equal to, or larger than, its inflation pressure. When the counter pressure is equal to the inflation pressure in the sealing pad, the sealing pad has the same shape as it does in the uninflated state, i.e., cylinder shape. In this case, the gap described above on the pressure side is found along the entire length of the obturator pad, and it joins together with the gap on the other, non-pressure side.
Since the sealing pad always loses its sealing power whenever the counter pressure is equal to the inner pressure, the result is that the sealing pad, even at pressures lower than the internal pressure, begins to slip within the tube.
With the above observation in mind, the static friction between the outer jacket and the inner wall of the tube could be disregarded. A rough calculation of the static friction shows, however, that the sealing pad can remain in a fixed position even when the counter pressure is several times its internal pressure if (1) the counter pressure is prevented from acting on the jacket surface of the sealing pad, and (2) the gap which necessarily exists is prevented from increasing. That is to say, this is true if it is possible to prevent the counter pressure from decreasing the force with which the pad abuts against the inner wall of the tube to be sealed.